EP3244527B1 - Elektret-element, elektromechanischer wandler und verfahren zur herstellung eines elektret-elements - Google Patents

Elektret-element, elektromechanischer wandler und verfahren zur herstellung eines elektret-elements Download PDF

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Publication number
EP3244527B1
EP3244527B1 EP16749241.2A EP16749241A EP3244527B1 EP 3244527 B1 EP3244527 B1 EP 3244527B1 EP 16749241 A EP16749241 A EP 16749241A EP 3244527 B1 EP3244527 B1 EP 3244527B1
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EP
European Patent Office
Prior art keywords
layer
sio
electret
electrode
comb
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English (en)
French (fr)
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EP3244527A4 (de
EP3244527A1 (de
Inventor
Hiroyuki Fujita
Gen Hashiguchi
Hisayuki Ashizawa
Hiroyuki Mitsuya
Kazunori Ishibashi
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University of Tokyo NUC
Saginomiya Seisakusho Inc
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University of Tokyo NUC
Saginomiya Seisakusho Inc
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
    • H02N1/00—Electrostatic generators or motors using a solid moving electrostatic charge carrier
    • H02N1/06—Influence generators
    • H02N1/10—Influence generators with non-conductive charge carrier
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
    • H02N1/00—Electrostatic generators or motors using a solid moving electrostatic charge carrier
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G7/00—Capacitors in which the capacitance is varied by non-mechanical means; Processes of their manufacture
    • H01G7/02—Electrets, i.e. having a permanently-polarised dielectric
    • H01G7/025—Electrets, i.e. having a permanently-polarised dielectric having an inorganic dielectric
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
    • H02N1/00—Electrostatic generators or motors using a solid moving electrostatic charge carrier
    • H02N1/002—Electrostatic motors
    • H02N1/006—Electrostatic motors of the gap-closing type
    • H02N1/008—Laterally driven motors, e.g. of the comb-drive type
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T29/00—Metal working
    • Y10T29/49—Method of mechanical manufacture
    • Y10T29/49002—Electrical device making
    • Y10T29/49226—Electret making

Definitions

  • the present invention relates to an electret element, an electromechanical converter and a method for manufacturing an electret element.
  • ions of an alkali metal such as potassium ions, which has been injected in an SiO 2 layer may be caused to move and fixed by applying a bias voltage at high temperature (see, for instance, PTL3).
  • the method described in PTL2 requires a state in which an electric field is applied to the charge-target area to be sustained during the charge processing. For instance, in order to charge comb teeth to their roots, the comb teeth of one must be fully inserted into the gaps between the teeth of the other and remain so inserted. However, as the charge processing progresses, the electrostatic force decreases, resulting in a decrease in the extent of teeth insertion. This means that a special mechanism for holding the comb teeth is required in order to sustain the extent of insertion. In addition, since the processing requires air, a sealed area cannot be effectively charged.
  • the electric charge is injected from the surface, making it difficult to control the position at which the charge is fixed (the depth from the surface).
  • a uniform charge cannot be achieved at a position deep into an insulating material.
  • An electric charge fixed near the surface will tend to become neutralized through a reaction with water vapor in the air and thus, the service life of the electret will be shortened.
  • ions of an alkali metal are used in the method disclosed in PTL3, alkali metals, which degrade the electrical characteristics of semiconductor elements, are normally kept out of a manufacturing apparatus. This means that since an electret cannot be formed in part of a CMOS device by using this method, the method is bound to limit the application range. In addition, since ions of an alkali metal are fixed at positions close to the SiO 2 surface, the method requires additional processing such as water-repellent film formation processing, in order to ensure the service life of the electret is not shortened.
  • PTL4 discloses an electret element according to the preamble of claim 1.
  • an electret element in accordance with claim 1 is provided.
  • an electromechanical converter in accordance with claim 2 is provided.
  • the Si layer is constituted with an Si substrate; and at least a part of a circuit element used to drive the electromechanical converter is formed at the Si substrate.
  • the electromechanical converter according to the second or third aspect it is preferred that power is generated as at least either the first electrode or the second electrode is caused to move by an external force.
  • the electromechanical converter in the electromechanical converter according to the second or third aspect, it is preferred that to further comprises a stationary unit having the first electrode disposed thereat, a movable unit having the second electrode disposed thereat, a voltage source that applies a voltage between the first electrode and the second electrode; and a control unit that drives the movable unit by controlling the voltage applied by that voltage source.
  • an electret element that includes an electret assuring an outstanding service life can be provided.
  • the electret element according to the first embodiment includes an Si layer and an SiO 2 layer, which are formed through an interface, and an electret is formed near the interface of the SiO 2 layer side.
  • the present inventors discovered the electrical characteristics manifesting at the Si/SiO 2 interface as will be described below and formed an electret in the SiO 2 layer by making use of the electrical characteristics.
  • a test piece 100 in FIG. 1 includes an SiO 2 layer 102 formed at a surface of an Si layer 101 on one side thereof.
  • An Au layer 103 and an Au layer 104, which are to function as electrodes, are respectively formed at the Si layer 101 and the SiO 2 layer 102.
  • Si is heated to high temperature (approximately 500 to 700°C)
  • the intrinsic carrier concentration increases, resulting in a lower electric resistivity, and thus it can be regarded as a conductor.
  • SiO 2 which is an excellent insulator at room temperature, is known to be affected by thermally excited electrons at high temperature (approximately 500 to 700°C), to result in its electric resistivity being reduced to as low as the order of 10 4 ⁇ m (setting it substantially in the range of a semiconductor).
  • the present inventors manufactured the test piece 100 having the Si/SiO 2 interface, as shown in FIG. 1 , and investigated the electrical characteristics manifesting at the Si/SiO 2 interface at high temperature (approximately 610°C). It was revealed, as indicated by the relationship between an applied voltage V1 and an electric current i in FIG. 2 , that a rectifying effect similar to that in a Schottky junction, is achieved at the Si/SiO 2 interface in high temperature conditions.
  • FIG. 3 illustrates the principle of charge at the electret element according to the present embodiment.
  • a substrate e.g., an SOI (silicon-on-insulator) substrate
  • SiO 2 layer 201 interposed between Si layers 202 and 203
  • an electric double layer is formed on the two sides of an Si/SiO 2 interface 204.
  • Si layers impurities has been doped are normally used, they may be either the p-type or the n-type. As an alternative, Si layers with no impurity content may be used.
  • a rectifying effect manifests at an Si/SiO 2 interface in high temperature conditions.
  • a positive charge is accumulated where the Si layer 202 is present on one side of the upper Si/SiO 2 interface 204 and a negative charge is accumulated where the SiO 2 layer 201 is present on the other side of the Si/SiO 2 interface 204.
  • no electric double layer is formed at the lower Si/SiO 2 interface where the voltage is applied along a direction in which the electric current flows.
  • the temperature of the substrate 200 is reset to room temperature while the voltage application is sustained, i.e., as its temperature is lowered to a level at which the SiO 2 layer 201 regains its insulating property, the negative charge having been accumulated in the SiO 2 layer 201 side of the Si/SiO 2 interface 204 becomes trapped, unable to move.
  • the application of the voltage V1 stops and the Si layer 202 and the Si layer 203 become connected with each other part of the positive charge moves from the Si layer 202 to the Si layer 203, as illustrated in FIG. 4 .
  • the negative charge within the SiO 2 layer 201 having regained its insulating property remains trapped near the Si/SiO 2 interface 204 even after the application of the voltage V1 stops.
  • an electric field E is formed within the SiO 2 layer 201, as illustrated in FIG. 4 .
  • This electric field E is a field induced with an electret, and the potential difference between the Si/SiO 2 interface 204 and an Si/SiO 2 interface 205 is V1. Namely, an electret with the voltage V1 is formed.
  • FIG. 5 is a schematic illustration of the structure having an SiO 2 layer interposed between Si layers.
  • Surface charges Q2 and Q3 are charges that form the electric double layers shown in FIG. 4 . While a distance d between the surface charges Q2 and Q3 at the electric double layer is extremely small, the distance d in FIG. 5 is exaggerated for clarity and it is assumed that the surface charge Q2 is fixed at the position indicated in the figure within the SiO 2 layer 201.
  • FIG. 5 also shows a surface charge Q1 in the Si layer 203 and the entire structure is assumed to be electrically neutral. Thus, it is assumed that a non-zero potential is achieved only at the electric fields E1 and E2 within the SiO 2 layer 201 and that the potential difference between the Si layers 202 and 203, attributable to the electric fields E1 and E2, is V.
  • an electric double layer is formed at the Si/SiO 2 interface 204 and the voltage V1 concentrates in this electric double layer.
  • Q 2 ⁇ ⁇ 1 ⁇ S ⁇ V 1 / d
  • Equation (10) above expresses the extent to which the positive charge moves as the potential difference changes from V1 to V.
  • the potential difference V becomes equal to 0, as indicated in FIG. 7 , and thus, the surface charge Q1 in the Si layer 203 can be calculated as expressed in equation (11) below.
  • Q 1 ⁇ ⁇ 1 ⁇ S ⁇ V 1 / g + d
  • the value of the surface charge Q3, on the other hand, is represented by the sum of an electric charge -Q2 induced via the surface charge Q2 and an electric charge -Q1 attributable to an outflow of the very small charge Q1, as indicated in equation (6).
  • This allows to be represented by a basic conceptual image of an electric double layer ⁇ Q2, - Q2 ⁇ having a high charge density, with a small charge Q1 moving between the upper and lower Si layers in correspondence to the potential difference.
  • an electric field can be generated in a gap space by executing charge processing on a specific structure, as will be described later.
  • Such an electric field generated in the gap space will enable electromechanical conversion (conversion from electric energy to mechanical energy and vice versa), which will allow to be adopted in power generation, sensors, actuators and the like.
  • the electret element according to the first embodiment is adopted in a vibration energy harvesting device assuming a comb tooth structure, representing an example of an electromechanical converter.
  • FIG. 8 schematically illustrates the structure of a vibration energy harvesting device 300.
  • the vibration energy harvesting device 300 is formed by processing an SOI substrate through a semiconductor integrated circuit manufacturing technology similar to that for standard MEMS (e.g., deep etching through ICP-RIE).
  • the vibration energy harvesting device 300 includes a fixed comb-tooth electrode 302 and a movable comb-tooth electrode 303, both disposed upon a rectangular ringshaped pedestal 301.
  • the movable comb-tooth electrode 303 is elastically supported on the pedestal 301 via an elastic support portion 305.
  • the individual comb teeth of the movable comb-tooth electrode 303 are set with gaps between the comb teeth of the fixed comb-tooth electrode 302.
  • a weight 304 is disposed at the movable comb-tooth electrode 303.
  • the movable comb-tooth electrode 303 vibrates along the direction indicated by the arrow R.
  • a load 320 is connected between the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303. As will be described later, an electret is formed at the fixed comb-tooth electrode 302, and as an external force is applied to the vibration energy harvesting device 300 and vibration occurs at the movable comb-tooth electrode 303, electric power is generated.
  • an oxide film (SiO 2 layer), at which an electret is to be formed is formed (with its thickness t set to approximately 0.2 to 1 ⁇ m) through thermal oxidation at the surface of the layer Si (see FIG. 10 ). Subsequently, an electret is formed as in the first embodiment by fixing an electric charge in the oxide film.
  • the oxide film (SiO 2 layer) is formed at the surface of the Si layer through thermal oxidation in the present embodiment, it may be adopted in conjunction with an oxide film (SiO 2 layer) formed through any of various other oxide film-forming methods. For instance, it may be adopted in conjunction with an oxide film (SiO 2 layer) formed by depositing SiO 2 onto an Si layer through CVD.
  • FIG. 9 illustrates a phase prior to the oxide film formation in a sectional view taken along B1 - B1 in FIG. 8 .
  • the pedestal 301 is formed with a handle layer (Si) at the SOI substrate.
  • the fixed comb-tooth electrode 302 is formed with a device layer (Si) at the SOI substrate.
  • the portion indicated with reference sign 307 is an embedded oxide film (SiO 2 ), which is referred to as a BOX layer in the SOI substrate.
  • the movable comb-tooth electrode 303, the elastic support portion 305 and the weight 304 are formed with the device layer at the SOI substrate.
  • FIG. 10 presents a sectional view taken along B1 - B1 following the oxide film formation and the subsequent charge processing.
  • An oxide film 310 is formed at the surface of the fixed comb-tooth electrode 302, formed with an Si layer and also over the surface of the pedestal 301, formed with an Si layer.
  • the charge processing for the oxide film 310 is executed as in the first embodiment by heating, with a heater or the like, the oxide film 310 constituting the SiO 2 layer until it reaches a temperature at which it is rendered into a semiconductor state. Once the oxide film 310 is rendered into the semiconductor state, it is cooled down to a temperature at which it regains its insulating property while applying a bias voltage V1 (10 to 200 V).
  • V1 bias voltage
  • the edges of the Si layer take on an R shape due to thermal oxidation, which lessens the extent of electric field concentration during the bias voltage application and thus increases the dielectric breakdown strength. For this reason, a relatively high bias voltage can be applied even though the gaps between the comb teeth of the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303 are small (approximately 2 ⁇ m).
  • the bias voltage V1 is applied between the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303, and between the fixed comb-tooth electrode 302 and also the bias voltage V1 is applied the pedestal 301, as illustrated in FIG. 11 .
  • the vibration energy harvesting device 300 is heated until its temperature is raised to a level (500 to 700°C) at which the oxide film 310 constituted of SiO 2 is rendered into a semiconductor state.
  • the bias voltage V1 is applied so as to form an electric double layer on the two sides of an Si/SiO 2 interface 306 at the fixed comb-tooth electrode 302.
  • FIG. 12 is a schematic sectional view (a section ranging parallel to the drawing sheet on which FIG. 11 is presented) of a region where the fixed comb-tooth electrode 302 with the electric double layer formed thereat and the movable comb-tooth electrode 303 overlap each other.
  • the oxide film formed at the fixed comb-tooth electrode 302 will be denoted with reference sign 310a and the oxide film formed at the movable comb-tooth electrode 303 will be denoted with reference sign 310b.
  • the Si layer of the fixed comb-tooth electrode 302 will be denoted with reference sign 311a and the Si layer at the movable comb-tooth electrode 303 will be denoted with reference sign 311b.
  • the potential difference at the electric double layer formed at the Si/SiO 2 interface 306 gradually increases until it eventually becomes equal to the voltage V1 (within several seconds to several minutes).
  • FIG. 13 is a schematic diagram corresponding to FIG. 5 pertaining to the first embodiment, illustrating in detail the structure assumed in the region enclosed by the dotted line C in FIG. 12 .
  • Surface charges Q5 and Q6, constituting the electric double layer on the two sides of the Si/SiO 2 interface 306, are formed respectively in the oxide film 310a and in the Si layer 311a at the fixed comb-tooth electrode 302.
  • a surface charge Q4 is an electric charge accumulated in the Si layer 311b at the movable comb-tooth electrode 303.
  • E3 is an electric field formed within the oxide film 310b at the movable comb-tooth electrode 303.
  • E5 and E6 are electric fields formed within the oxide film 310a at the fixed comb-tooth electrode 302.
  • E4 is an electric field formed in a gap space G between the comb-tooth electrodes 302 and 303.
  • Equation (13) through (17) can be written by adopting Gauss's Law individually for the region where the surface charge Q4 is present, the region that includes the interface of the oxide film 310b and the gap space G, the region that includes the interface of the oxide film 310a and the gap space G, the region where the surface charge Q5 is present and the region where the surface charge Q6 is present, as illustrated in FIG. 13 .
  • S represents the area of a section of a region C in FIG. 12 .
  • ⁇ 0 and ⁇ 1 respectively represent the dielectric constant in the gap space G and the dielectric constant in the oxide film (SiO 2 ).
  • the surface charge Q5 i.e., an electret charge
  • Q 5 ⁇ d + g 1 + g 2 ⁇ 1 / ⁇ 0 + g 3 / d
  • Q 4 ⁇ ⁇ 1 ⁇ S ⁇ V / d
  • the surface charge Q5 accumulated in the oxide film 310a becomes fixed at the position indicated in FIG. 14 .
  • the Si layer 311a in the fixed comb-tooth electrode 302 and the Si layer 311b in the movable comb-tooth electrode 303 are connected as shown in FIG. 15 , the charge (Q4) is caused to move from the Si layer 311a to the Si layer 311b due to the potential difference (see FIG.
  • FIG. 16 schematically illustrates a state (c) achieved as the movable comb-tooth electrode 303 slides away from the fixed comb-tooth electrode 302 until their comb teeth do not overlap at all, a state (b) in which their comb teeth are half overlapped and a state (a) in which the comb teeth entirely overlap.
  • This operational rationale may be conceptualized in conjunction with a load 320 having an impedance set at the lower limit connected therein, which corresponds to a condition in which the charge quantity of the surface charge Q4 changes as the area S (equivalent to the overlap area) changes in the equation (22).
  • each negative sign in the surface charge Q5 represents a charge quantity -q and each positive sign in the surface charges Q4 and Q6 represents a charge quantity +q for purposes of simplification.
  • the potential in the Si layer 311a at the fixed comb-tooth electrode 302 and the potential in the Si layer 311b at the movable comb-tooth electrode 303 are equal to each other. Namely, the potential difference V is equal to 0. This means that no electric current flows through the load 320.
  • the surface charge Q6 achieves a charge quantity +6q
  • the surface charge Q5 achieves a charge quantity -8q
  • the surface charge Q4 achieves a charge quantity +2q.
  • the charge quantity of the surface charge Q4 also decreases. Then, when the overlap area becomes 0 in the state (c), the charge quantity of the surface charge Q4 also becomes 0 whereas the surface charge Q6 achieves a charge quantity +8q.
  • FIG. 17 schematically illustrates the structure of an MEMS shutter 400 according to the embodiment.
  • the MEMS shutter 400 is formed by processing an SOI substrate and includes a fixed comb-tooth electrode 302 secured at a pedestal 301 taking the shape of a rectangular ring and a movable comb-tooth electrode 303 secured at the pedestal 301 via an elastic support portion 305.
  • the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303 configure a comb-tooth actuator.
  • the shutter unit 404 with an opening 404a formed thereat is disposed at the movable comb-tooth electrode 303.
  • a voltage to be used for actuator drive is applied between the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303.
  • a control unit 402 causes the movable comb-tooth electrode 303 with the shutter unit 404 disposed thereat to move along a direction indicated by the arrow R by controlling the voltage V applied from the voltage source 401.
  • the shutter unit 404 is positioned on an optical path, and as the movable comb-tooth electrode 303 moves and the opening 404a at the shutter unit 404 is set in the optical path, a light beam passes through the shutter unit 404. When the non-open region (shielding region) of the shutter unit 404 is set in the optical path, the light beam is blocked by the shutter unit 404.
  • the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303 assume structures similar to those described in reference to the second embodiment, are formed through methods similar to those described earlier in reference to the second embodiment and an electret is formed at the fixed comb-tooth electrode 302 through a method similar to that described in reference to the second embodiment, a repeated explanation is not provided.
  • FIGs. 18 through 20 illustrate how the comb-tooth actuator is engaged in drive operation.
  • FIG. 18 illustrates a state in which the voltage V applied from the voltage source 401 is 0.
  • (a) shows forces F1 and F2 act to the movable comb-tooth electrode 303 and (b) indicates the relationship between the applied voltage V and an electric field E4.
  • the applied voltage V is 0, the potential at the Si layer 311a and the potential at the Si layer 311b are equal to each other and thus, a state identical to that shown in FIG. 15 and FIG. 16 is achieved.
  • the electric field E4 which can be calculated as expressed in equation (24) explained earlier, is formed in the gap space G between the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303. Via this electric field E4, the force F1, working along the rightward direction in the figure, is applied to the movable comb-tooth electrode 303 so as to draw its comb teeth further into the gaps between the comb teeth in the fixed comb-tooth electrode 302.
  • the elastic support portion 305 alters its shape, as illustrated in FIG. 18(a) .
  • a force F2 attributable to the elastic force imparted by the elastic support portion 305 is applied to the movable comb-tooth electrode 303 so as to draw it back to the left in the figure.
  • the movable comb-tooth electrode 303 comes to a stop at a position at which the force F1 and the force F2 are in balance.
  • FIG. 19 illustrates a state in which the applied voltage V is set so that 0 ⁇ V ⁇ V1.
  • the value representing the electric field E4 in the gap space G in this state can be expressed as in equation (25) below, which can be obtained by adopting equations (13) and (14) in equation (22) explained earlier.
  • equations (24) and (25) indicate, the intensity of the electric field E4 in FIG. 19 is lower than that achieved when the applied voltage V is 0.
  • FIG. 20 illustrates a state in which the applied voltage V is set to V1.
  • the charge quantity of the surface charge Q5 and the charge quantity of the surface charge Q6 are equal to each other and the potential difference equal to V1 manifests at the electric double layer formed with those charges.
  • no electric field E4 is generated in the gap space G and accordingly, no electrostatic force F1 is in effect between the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303. This means that no deformation occurs at the elastic support portion 305, as illustrated in FIG. 20 .
  • a shutter can be opened/closed via the shutter unit 404 by altering the voltage V applied from the voltage source 401 so as to drive the movable comb-tooth electrode 303 in a sliding motion.
  • the presence of an electret fitted in a comb-tooth electrode as shown in FIGs. 18 through 20 , makes it possible to achieve the highest level of intensity for the electric field E4 in the gap space G when the applied voltage V is 0.
  • the electrostatic force in action between the comb teeth in a comb-tooth actuator is in proportion to the square of the electric field. Accordingly, the relationship between the applied voltage V and the electrostatic force F1 achieved in a structure in which the comb-tooth actuator is driven entirely with the applied voltage V without using an electret is represented by a quadratic curve such as a line L1 in FIG. 21 .
  • the relationship between the applied voltage V and the electrostatic force F1 achieved in a comb-tooth actuator having an electret formed therein as in the embodiment, on the other hand, may be represented by, for instance, a line L2.
  • the line L2 is offset relative to the line L1 toward the positive side along the horizontal axis by an extent corresponding to the charging voltage V1 at the electret.
  • an electrostatic force ⁇ Fb at the comb-tooth actuator with the electret with a given voltage ⁇ V applied thereto is greater than an electrostatic force ⁇ Fa generated at the comb-tooth actuator without an electret with the same voltage ⁇ V applied thereto.
  • a greater electrostatic force can be achieved in conjunction with an electret in comparison to that achieved in a structure engaged in operation entirely on an external bias voltage.
  • an electret element includes the Si layer 202, the SiO 2 layer 201 formed at the surface of the Si layer 202 and the electret (surface charge Q2) formed at the Si layer 201 near the interface of the SiO 2 layer 201 and the Si layer 202, as illustrated in FIGs. 4 and 7 . Since the surface charge Q2 constituting the electret is fixed near the Si/SiO 2 interface, the SiO 2 layer 201 functions as a protective film, making it possible to improve the service life of the electret.
  • the electret is formed by applying a voltage between the Si layer 202 and the SiO 2 layer 201 while sustaining the Si layer 202 with the SiO 2 layer 201 formed thereat at a first temperature (approximately 500 to 700°C) at which the SiO 2 layer 201 is rendered in a semiconductor state and then by altering the temperature of the Si layer 202 with the SiO 2 layer 201 formed thereat from the first temperature to a second temperature (e.g., approximately 300°C or lower) as which the SiO 2 layer 201 regains its insulating property while applying the voltage continuously.
  • a first temperature approximately 500 to 700°C
  • a second temperature e.g., approximately 300°C or lower
  • an electret can be formed with ease even in a narrow gap region such as along the side surfaces of the comb teeth of a comb-tooth electrode, as shown in FIG. 8 , or at an electrode located in a sealed space. Since an electret can be formed with ease over a narrow gap region, the dimensions of the gaps can be further reduced, which, in turn, makes it possible to improve the performance of a power generation device or an actuator.
  • the second embodiment includes the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303 disposed so as to face opposite each other with the fixed comb-tooth electrode 302 constituted with an electret element.
  • the device structured as in the second embodiment is able to function as the electromechanical converter (e.g., the vibration energy harvesting device 300) capable of converting electric energy to mechanical energy and vice versa, as the movable comb-tooth electrode 303 moves, i.e. as the movable comb-tooth electrode 303 becomes displaced relative to the fixed comb-tooth electrode 302.
  • the electromechanical converter e.g., the vibration energy harvesting device 300
  • an electret is formed at the fixed comb-tooth electrode 302 in the embodiments described above, it may be adopted in a structure with an electret formed at the movable comb-tooth electrode 303. Furthermore, it may be adopted in a structure in which a pair of comb-tooth electrodes are both allowed to move, as well as in a structure in which only either of the pair of comb-tooth electrodes is allowed to move.
  • An electromechanical converter may be realized as an actuator used to drive the shutter unit 404 shown in FIG. 17 or as an electret condenser microphone instead of as a power generation device.
  • the electret element according to the embodiments described above are distinguishable from the electret disclosed in PTL3 which holds alkali metal ions, and thus, it can be used in conjunction with a CMOS device.
  • it allows some of the circuit elements in the control unit 402 in FIG. 17 to be formed in the Si layer (device layer) at the pedestal 301.
  • Such circuit elements include, for instance, transistors for driving circuit, FETS and resistors for a microphone amplifier circuit or a sensor amplifier circuit, and rectifier diodes for a power generation element.
  • electrodes 302 and 303 in the second embodiment described earlier assume a comb structure, they may instead assume a parallel plate structure with a variable gap distance.
  • An electret element with such parallel plate electrodes can be used in applications as a parallel plate vibration energy harvesting device or a parallel plate condenser microphone.
  • the charge processing is executed in the embodiments described above by heating the entire device that includes the fixed comb-tooth electrode 302 and the movable comb-tooth electrode 303, only the regions involved in the electret formation (i.e., the SiO 2 layer to be charged and the Si layer through which an electric current is to flow) may be selectively heated with a laser or the like.
  • the present invention can be adopted in a device such as an electret microphone with a built-in amplifier circuit.

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Claims (6)

  1. Elektret-Element, umfassend:
    eine Si-Schicht (101, 202, 203, 311a, 311b),
    eine SiO2-Schicht (102, 201), die an einer Oberfläche der Si-Schicht ausgebildet ist; und
    ein Elektret, das an der SiO2-Schicht in der Nähe einer Grenzfläche (204, 205, 306, 308) der SiO2-Schicht und der Si-Schicht ausgebildet ist, dadurch gekennzeichnet, dass:
    die Oberflächenladung, die das Elektret bildet, in der Nähe der Grenzfläche (204, 205, 306, 308) fixiert ist; und die SiO2-Schicht (102, 201) als ein Schutzfilm des Elektrets fungiert.
  2. Elektromechanischer Wandler (300, 400), der eine erste Elektrode (302) und eine zweite Elektrode (303) vorsieht, die so angeordnet sind, dass sie einander gegenüberliegen, wobei es mindestens einer davon erlaubt ist, sich zu bewegen, wobei:
    die erste Elektrode (302) mit dem Elektret-Element gemäß Anspruch 1 gebildet ist; und
    elektrische Energie in mechanische Energie umgewandelt wird und umgekehrt, wenn sich mindestens entweder die erste Elektrode oder die zweite Elektrode bewegt.
  3. Elektromechanischer Wandler (300, 400) gemäß Anspruch 2, wobei:
    die Si-Schicht mit einem Si-Substrat (200) gebildet ist; und
    mindestens ein Teil eines Schaltkreiselements, das zum Ansteuern des elektromechanischen Wandlers verwendet wird, an dem Si-Substrat ausgebildet ist.
  4. Elektromechanischer Wandler (300, 400) gemäß Anspruch 2 oder Anspruch 3, wobei:
    Strom erzeugt wird, wenn entweder die erste Elektrode (302) oder die zweite Elektrode (303) durch eine externe Kraft in Bewegung versetzt wird.
  5. Elektromechanischer Wandler (400) gemäß Anspruch 2 oder Anspruch 3, ferner umfassend:
    eine stationäre Einheit, an der die erste Elektrode (302) angeordnet ist,
    eine bewegliche Einheit, an der die zweite Elektrode (303) angeordnet ist,
    eine Spannungsquelle (401), die eine Spannung zwischen der ersten Elektrode und der zweiten Elektrode anlegt; und
    eine Steuerungseinheit (402), die die bewegliche Einheit durch Steuern der von der Spannungsquelle angelegten Spannung ansteuert.
  6. Verfahren zum Herstellen eines Elektret-Elements, umfassend:
    Anlegen einer Spannung zwischen einer Si-Schicht (101, 202, 203, 311a, 311b), an der eine SiO2-Schicht ausgebildet ist, und der SiO2-Schicht, während die Si-Schicht auf einer ersten Temperatur gehalten wird, bei der die SiO2-Schicht in einen Halbleiterzustand versetzt wird; und
    Ändern von Temperaturen an der Si-Schicht, an der die SiO2-Schicht ausgebildet ist, von der ersten Temperatur in eine zweite Temperatur, bei der die SiO2-Schicht eine isolierende Eigenschaft zurückerhält, in einem Zustand eines kontinuierlichen Anlegens von Spannung, wobei:
    ein Elektret in der Nähe einer Grenzfläche (204, 205, 306, 308) der SiO2-Schicht und der Si-Schicht ausgebildet wird; die Oberflächenladung, die das Elektret bildet, in der Nähe der Grenzfläche fixiert ist; und die SiO2-Schicht als ein Schutzfilm des Elektrets fungiert.
EP16749241.2A 2015-02-13 2016-02-09 Elektret-element, elektromechanischer wandler und verfahren zur herstellung eines elektret-elements Active EP3244527B1 (de)

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JP7249597B2 (ja) * 2020-03-27 2023-03-31 国立大学法人 東京大学 発電素子の製造方法、及び、発電素子
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